1. Field of the Invention
The present invention generally relates to high-speed optoelectronic transducers. In particular, some example embodiments relate to mesa-type optical detectors that include a protection layer substantially covering a passivation layer and configured to substantially prevent degradation of the passivation layer.
2. Related Technology
Optical detectors are commonly employed for converting optical signals to electrical signals in a variety of applications. Such optical detectors are used, for instance, in connection with optical transmitters to monitor the output power of an optical signal produced by the optical transmitter, and as optical receivers to convert an incoming optical data signal into an electrical data signal in a communications network.
Optical receivers in communications networks are typically configured to operate at certain signaling rates, such as 1 Gigabit per second (“Gbit/s”), 2 Gbit/s, 4 Gbit/s, 10 Gbit/s, 40 Gbit/s, 100 Gbit/s, and above. The highest signaling rate at which an optical receiver can operate at is limited by the capacitance of the optical receiver, which is related to the size of the optical receiver's active region. In particular, higher capacitance in optical receivers with relatively larger active regions limits the use of such optical receivers to low-speed applications. In contrast, lower capacitance in optical receivers with relatively smaller active regions allows such optical receivers to be used in high-speed applications. In other words, faster data rates require optical receivers with smaller active regions.
One type of optical receiver adapted for high-speed applications is the mesa-type optical detector. The active region of a mesa-type optical detector can be minimized by forming a multi-layered semiconductor structure on a substrate and etching through various layers of the semiconductor structure to form a mesa. The mesa thus formed defines the diameter of the optical detector's active region, which at least partially determines the data rate capabilities of the optical detector. By way of example, the active region diameter of some 4 Gbit/s optical detectors is approximately 50 microns, while the active region diameter of some 10 Gbit/s optical detectors is approximately 30 microns.
In the course of forming the mesa, defects and/or discontinuities are typically created in the layers of the semiconductor structure along the mesa line. Exposure of the mesa surfaces to air oxidizes the layers and creates more defects and/or discontinuities. The defects and/or discontinuities can be a source of dark current, e.g., electric current that flows through an optical detector even when no photons are entering the active region of the optical detector. The dark current adds noise to photocurrent generated during operation of the optical detector and can render an optical detector useless if it becomes too large.
To minimize dark current, a passivation layer is typically formed on the mesa as soon as possible after etching the mesa during the manufacturing process. Ideally, the passivation layer prevents the deterioration of the layers of the semiconductor structure along the mesa line and thereby stabilizes the dark current to a manageable level. In practice, the passivation layer is sensitive to air, moisture, temperature swings, chemicals, mechanical stress, and/or other external factors that degrade the passivation layer. The resulting degradation of the passivation layer allows the mesa to deteriorate, reducing the ability of the passivation layer to minimize dark current and thereby reducing the reliability of the optical detector.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.